7. What's Going On At UNIT?
Very quickly we see Tennant and Smith sit down and cross their legs almost identically. It's a quick reminder that despite their differences, they're the same character and we'll likely be getting our fair share of that in 'The Day of the Doctor'. During that shot, though, on both sides of the frame appears to be Kate Stewart, daughter of the late, great Brigadier. Identical clothes, almost identical people, save for slightly different hair styles. This has the potential to be anything from a minor plot point to a major one. We know the Zygons are involved in the special in some capacity, so perhaps Tennant and Smith quickly resolve a minor Zygon infiltration close to the start of the episode to showcase the might of two-Doctor problem solving. On the other hand, Doctor Who has not been shy when it comes to using parallel universes. Maybe it's a Kate Stewart from a different world, which would suggest the solution to the timeline problems when using Tennant and Smith together is that Tennant is from another world. Maybe even the Meta-crisis Doctor? Admittedly, if this is the case, things start to get very dependent on viewers remembering previous series information.